Nature of Underwater Explosions

An imderwater explosion whether chemical, gas, or electrical spark involves an initial outward acceleration of energy to the surroimding mediiun. A substantial portion of the total energy is not released immediately in the form of a compressional seismic wave, but is retained and stored temporarily as kinetic energy in the water. At a later time, the outward flow of water is reversed, and the inward flow produces a secondary seismic pulse called the first bubble pulse. 

The disturbance in seawater travels rapidly outward, creating a growing spherical cavity. The traveling gas-liquid interface is known as the detonation front. Seawater is not an ideal elastic solid but has a bulk modulus that is a fxmction of pressure. The seawater as it is compressed becomes stiffen Therefore, the velocity of the outward traveling shock wave is a function of the peak pressure of the new wave. As the wave travels outward, the peak pressure decreases as a negative power of radial distance from the source. The value of the negative power decreases from typically -3 to -1.3 with distance from the source (Kramer et al., 1968 Dobrin, 1976). When the initial positive pressure pulse reaches a free water surface, it is reflected as a pulse of opposite polarity. 

The compressional wave tiiat is generated from tiiis process travels away from the source in all directions xmtil it is partially reflected upward, parhally refracted, and partially transmitted downward into the seafloor at each layer interface where there is a variation in seismic impedance. Seismic impedance is defined as follows  

Vi is the compressional velocity layer 1. V2 is the compressional velocity layer 2. 

As the amount of contrast increases as reflected by an increasing impedance ratio (af), the less energy is transmitted. The limit of penetration is reached when the reflected energy is too weak to be detected. 

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